The dot is a part of the extension, i.e. use as
fp.ExtensionIs(".txt")
bool ExtensionIs(
string extension
)[Missing <param name="extension"/> documentation for "M:DHI.Mike1D.Generic.IFilePath.ExtensionIs(System.String)"]
[Missing <returns> documentation for "M:DHI.Mike1D.Generic.IFilePath.ExtensionIs(System.String)"]